Guitar Finishes: The 'Polys'
After our tour of nitrocellulose last week, I figured it would make sense to take a look at some of the more modern finishes.
The world of finishing is broader and deeper than many might imagine and, despite my having to work with this stuff, there are still plenty of times I have to pause and double-check definitions and specifications. The term 'lacquer' is often used as a catch-all but, incorrectly. The poly-finishes cover a few different types with different bases. It's confusing and more than a little messy for an outsider so we'll try to limit ourselves to a few guitar-related things.
First off, I have to define some stuff:
Nitrocellulose is a solvent-based finish. As it dries, the solvents evaporate, leaving behind a film of solid material. That material was once suspended/dissolved in those solvents. After the initial curing process, further solvent evaporation can occur very slowly over longer periods. Also, the solids in the finish can be re-dissolved if they come in contact with a similar solvent.
The finishes (polyurethane and polyester) we're looking at first are different. They're reactive finishes, meaning they cure chemically. There are two main parts to the finish—the resin and a catalyst. As the finish cures, these react chemically and the molecules link together (cross-linking) to form a strong polymer, essentially a plastic*. The cross-linking process forms a very tough film with no real 'spaces', which means it's pretty much impervious to liquids.
Reactive finishes have very small amounts of solvent or thinners and this does evaporate after spraying. There's much less solvent than nitrocellulose, though, and there's an important difference: Once cured, the reactive finish has formed a film that cannot be re-dissolved by applying solvent.
Polyurethane and Polyester
We've all probably heard the term 'poly finish' to describe a particular type of guitar coating. That's another catch-all but, in our world, mainly applies to either polyurethane or polyester finishes.
Polyurethane began being used by Fender sometime in the late '60s. As with most specification decisions, it worked out cheaper. It could be applied more quickly—a couple of layers of polyurethane instead of multiple coats of lacquer, it dried faster, and it could be buffed to a gloss with much less trouble. And, in an age where yellowed and cracked finishes were still seen as a bad thing, polyurethane provided a coating that didn't discolour or check. It also stood up to the rigours of being used by musicians: It was tougher, didn't scratch or wear so easily and even laughed off splashes of water, beer, and various liquors.
Some time in the early '70s, polyester strode into Fender's spray rooms, with an eye to knock polyurethane off the throne. And it succeeded. In part.
Polyester is a similar finish to polyurethane. It requires more specialised equipment to apply because it has a significantly higher solids content. However, that high solids content means that it builds to a finish much more quickly (and it has even less environmental impact from solvent evaporation than polyurethane). Polyester is particularly well suited to wood finishing and it's even more tough. The king is dead, long live the king.
But something went wrong. People didn't like these new finishes.
"My tone! My tone!" cried guitarists around the world and there was much weeping and gnashing of teeth. Turns out guitars from the big builders—once the pinnacle of sought-after tone—were not sounding so good as before.
"It must be this thick and shiny finish to blame," yelled someone, and the cry was taken up. "Guitars must breathe and resonate and poly is killing our tone."
Probably not, though. While there's no doubt that some of the poly (both polys) finishes from the time were thick and plasticky, it's debatable whether that was impacting the instrument's tone. Far more likely that the penny-pinching going on in all the other departments was affecting how the finished guitar sounded. In particular, specifications for pickup components and wire were mostly ignored in the quest for cost-savings. Personally, my money's on that stuff rather than the finish.
So what was the deal with those heavy, thick finishes? The story goes (and it seems to add up) that the powers-that-be threw new finishes at the spray crews without really considering whether any change in working practice was needed. If you're used to spraying a dozen or more coats of nitro to get a decent build and you're suddenly using a high-solids finish that can get the same build in two coats, there's going to be an adjustment phase.
Are poly's bad for tone? No. They're not. Fender are still using polyurethane and polyester on a heap of guitars today and they sound great. The finishers, long ago, figured out how to properly apply these for thinner coats. Across the world, and across the price-spectrum, builders are using polys on guitars. Some of these instruments probably sound awful but some sound fantastic. It's not the finish that's responsible for that.
Thing is, accepted wisdom is a strange thing. It becomes a sort of folk memory and can take generations to fade away. I'm on record as loving a wonderfully checked and aged finish but I don't think that poly is bad and nitrocellulose is magic. I like the way nitro looks but I know for a fact (because I deal with them often) that there are many guitarists who like their guitars to be pristine, scratch free, and shiny. And they prefer them to stay that way for ever. Polys (and some other modern finishes) are these players' friends.
For my part, as a repairer, the qualities that make polyurethane and polyester fantastic for someone applying them—or someone using the guitar they're applied to—make them a pain in the ass for me. That tough chemical bond means the finish cannot be dissolved. There is no way for a new coat to 'bond' with a previous coat. So, any new finish really just sits on top of the existing finish. That makes repair and touch-up much more tricky. You've got to be careful to get good adhesion and you'll always have 'witness lines' between the new and old finish. Invisible repairs are not really a runner.
But it's not all about me. 😉 Repairability is less and less a factor in modern manufacturing and there's no point in my trying to order the tide to remain out.
In my view, well applied poly finishes make for fantastic looking guitars and have no discernible affect on the instrument's tone. As you'd expect, these days there are some new finish contenders but polys are still used, in various forms, on countless guitars.
*Nitrocellulose purists thinking about the 'plastic' term should recall that nitro is a plastic too. One of the first plastics, actually.
This article written by Gerry Hayes and first published at hazeguitars.com